The present invention relates to an optical filter. More particularly, the present invention relates to an optical filter made of synthetic resins, which is capable of efficiently cutting-off light in the near infrared region and has light absorbing characteristics favorable for the adjustment of luminous efficiency.
Hitherto, the optical filters made of specific phosphate glass incorporated with copper ions have been used as photometric filters or luminous efficiency filters for cameras.
These optical filters made of glass, however, have involved many problems such as heavy weight, high susceptibility to devitrification with the passage of time due to high hygroscopicity, and difficulties in processing such as molding, cutting and polishing in the manufacture of the filters.
Optical filters made of synthetic resins have been proposed for solving the problem of "heavy weight".
U.S. Pat. No. 4,152,332 proposes an infrared absorber composed of a metal complex of a bis-[cis-1,2-bis-(alkyl, hydrogen, aryl or heterocyclic) ethylene-1,2-dithiolate].
JP-A-61-134702 proposes a near infrared absorbing filter having as its constitutional element an organic layer containing at least one of the compounds represented by the formula: ##STR2## wherein R.sub.1 is an aromatic group and R.sub.2 is a hydrogen atom or an aromatic group.
JP-A-60-184542 proposes a methacrylic resin composition for infrared filters comprising 100 parts by weight of a methacrylic resin and 1.2 to 4.0 parts by weight of Solvent Green 28 which is an anthraquinone dye.
JP-A-61-32003 proposes an optical filter material containing at least one of the compounds represented by the following formula: ##STR3## wherein M is Cu, Co, Ni, Pd or Pt; Ca.sup.+ denotes a cation which neutralizes the complex; and R.sup.1, R.sup.2, R.sup.3 and R.sup.4 each represents a hydrogen atom, a halogen atom, an amino group, a cyano group, a hydroxyl group, or an alkyl group, an aryl group or a heterocyclic group in which a divalent coupling group may exist between the benzene ring to be bonded and such group, at least one of these groups may be an electron donative group, and these groups may be the same or different from each other.
JP-A-61-20002 proposes an optical filter material containing at least one of the organo-metallic compounds represented by the following formula: ##STR4## wherein R.sup.1, R.sup.2, R.sup.3 and R.sup.4 are each a hydrogen atom a halogen atom, a cyano group, a hydroxyl group, or an alkyl group, an aryl group, a cycloalkyl group or a heterocyclic group in which a divalent coupling group may exist between the benzene ring to be bonded and such group, or R.sup.1 and R.sup.2, R.sup.2 and R.sup.3 or R.sup.3 and R.sup.4 designate the non-metallic atomic groups which are bonded to each other to form a 5- or 6-member ring, and the groups the R.sup.1 to R.sup.4 group may be the same or different from each other; and X denotes an anion which neutralizes the cation in the above formula.
Any of these filters, however, fails to satisfy all of the quality and performance requirements for an optical filter, such as high cut-off efficiency for light in the near infrared region, low hygroscopicity and good processability. Therefore, it is strongly required to provide an optical filter which is light in weight, low in hygroscopicity and good in processability.
As the result of the present inventors' extensive researches on the subject matter, it has been found that by molding a composition prepared by adding a metallic compound mainly composed of a copper compound to a resin material obtained by copolymerizing a specific monomer having a phosphorous group with another monomer copolymerizable therewith, the obtained optical filter made of synthetic resins is capable of efficiently cutting-off light in the near infrared region, low in hygroscopicity, proof against devitrification with the passage of time and light in weight, and is suited for use as a photometric filter or a luminous efficiency filter for cameras. The present invention has been attained on the basis of this finding.